Intergalactic star

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The Virgo cluster of galaxies, where the phenomenon known as intergalactic stars was discovered.

An intergalactic star, also known as a rogue star, is a star not gravitationally bound to a galaxy. These stars were a source of much discussion in the scientific community during the late 1990s, and are generally thought to be the result either of galaxies colliding, or of a binary star pair travelling too close to a black hole (which can be found at the center of many galaxies).


The common belief that stars exist only in galaxies was disproven in 1997 with the discovery of intergalactic stars.[1] The first to be discovered were in the Virgo cluster of galaxies, where some one trillion are now surmised to exist.[2]

The discovery of intergalactic stars addresses the "missing baryon problem"[3]—baryons are a class of subatomic particles including the protons and neutrons that make up the cores of the atoms in normal matter, but astrophysicists and cosmologists usually refer to any ordinary matter as baryons, thus including electrons. The missing baryon problem is identified by many theories of formation and evolution of the universe, which point out that there should be many more baryons than those that scientists have found.[4]


The way these stars arise is still a mystery, but the most common theory is that the collision of two or more galaxies can toss some stars out into the vast empty regions of intergalactic space. Although stars normally reside within galaxies, they can be extracted by gravitational forces when galaxies collide. It is commonly believed that these intergalactic stars may also have come from extremely small galaxies, since it is easier for stars to escape a smaller galaxy's gravitational pull.[5]

A collision between galaxies is commonly thought to be the source of intergalactic stars.

Another theory (an example of which is shown in the image below) states that stars can be ejected from their galaxy by supermassive black holes. In this theory, it's likely that the soon-to-be intergalactic star is a part of a binary star system where one of the stars is pulled into the supermassive black hole at the center of a galaxy and the other is ejected away after travelling too close to the aforementioned black hole. The star that has been ejected from the galaxy's center is travelling at high speeds to escape the gravitational pull of the galaxy, and is now called a hypervelocity star.[6]

Proposed mechanisms for the ejection of intergalactic stars by supermassive black holes.

Observation history[edit]

In 1997, the Hubble telescope discovered a large number of intergalactic stars in the Virgo cluster of galaxies. Later in the 1990s scientists discovered another group of intergalactic stars in the Fornax cluster of galaxies.

Recently astronomers have noticed a faint glow that may be a product of intergalactic stars. This discovery suggests that nearly half of the entirety of stars in the universe may reside outside the known galaxies: "The findings come from the Cosmic Infrared Background Experiment (CIBER), which flew briefly into space in 2010 and 2012 aboard a sounding rocket. As CIBER soared above the atmosphere, it looked at five different regions in space for about a minute each, gathering as many particles of cosmic light as possible. The flights took place at different times of year [...] But when Zemcov and his colleagues began to sift through CIBER's data, they realized that the light it captured was not nearly red enough to have come from ancient galaxies. The light must be coming from something closer and more modern, they say—such as ordinary stars."[7]

Some Vanderbilt astronomers report that they have identified more than 675 stars at the edge of the Milky Way, between the Andromeda Galaxy and the Milky Way. They argue that these stars are hypervelocity (intergalactic) stars that were ejected from the Milky Way's galactic center. These stars are red giants with a high "metallicity" (a measure of the proportion of chemical elements other than hydrogen and helium within a star) indicating an inner galactic origin, since stars outside the disks of galaxies tend to have low metallicity and are older.[8]

In 2005, at the Smithsonian Center for Astrophysics, Warren Brown and his team attempted to measure the speeds of hypervelocity stars using the Doppler Technique, by which light is observed for the similar changes that occur in sound when an object is moving away or toward something. But the speeds found are only estimated minimums, as in reality their speeds may be larger than the speeds found by the researchers. "One of the newfound exiles is moving in the direction of the constellation Ursa Major at about 1.25 million mph with respect to the galaxy. It is 240,000 light-years away. The other is headed toward the constellation Cancer, outbound at 1.43 million miles per hour and 180,000 light-years away."[9]

Some recently discovered supernovae have been confirmed to have exploded hundreds of light years from the nearest star or galaxy.[10]


Although the precise mass of these stars cannot be known, it is estimated that together they make up 10 percent of the mass of the Virgo cluster of galaxies. This means that, most likely, these stars collectively have a larger mass than any particular one of the 2500 galaxies that form the Virgo cluster.[11]

Known locations[edit]

The first intergalactic stars were discovered in the Virgo cluster of galaxies. These stars form a massive group approximately 300,000 light years away from the nearest galaxy.

Approximately 675 rogue stars have been discovered at the edge of our galaxy, between the Andromeda galaxy and the Milky Way.[12]

See also[edit]


  1. ^ "NewsCenter - Hubble Finds Intergalactic Stars (01/14/1997) - Introduction". HubbleSite. 1997-01-14. Retrieved 2010-12-09. 
  2. ^ "NewsCenter - Hubble Finds Intergalactic Stars (01/14/1997) - Release Text". HubbleSite. 1997-01-14. Retrieved 2010-12-09. 
  3. ^ "Colossal Gas Cloud Discovered Around Milky Way". Space. Retrieved 3 January 2015. 
  4. ^ Choi, Charles Q. "Lost in Space: Half of All Stars Drifting Free of Galaxies". Space. Purch. Retrieved 3 January 2015. 
  5. ^ Witze, Alexandra. "Half of Stars Lurk Outside Galaxies". Nature. Nature Publishing Group. Retrieved 3 January 2015. 
  6. ^ Britt, Robert Roy. "Exiled stars: Milky Way Boots Members". USATODAY. Retrieved 4 January 2015. 
  7. ^ Witze, Alexandra. "Half of Stars Lurk Outside Galaxies". Nature. Nature Publishing Group. Retrieved 3 January 2015. 
  8. ^ Salisbury, David. "Rogue Stars Ejected From the Galaxy Found in Intergalactic Space". Vanderbilt University. Vanderbilt University. Retrieved 3 January 2015. 
  9. ^ Britt, Robert Roy. "Exiled Stars: Milky Way Boots Members". USATODAY. Retrieved 4 January 2015. 
  10. ^ Stallard, Brian. "Between Galaxies: Lonely Supernovae Identified". Nature World News. 
  11. ^ Henry C. Ferguson; et al. (1998), "Detection of intergalactic red-giant-branch stars in the Virgo cluster", Nature, 391: 461–463, Bibcode:1998Natur.391..461F, arXiv:astro-ph/9801228Freely accessible, doi:10.1038/35087 
  12. ^ Salisbury, David. "Rogue Stars Ejected From the Galaxy Found in Intergalactic Space". Vanderbilt University. Vanderbilt University. Retrieved 3 January 2015.